1. Field of the Invention
The invention is generally related to the area of optical components. In particular, the invention is related to compact wavelength multiplexer/demultiplexer and method for making the same.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. WDM (Wavelength Division Multiplexing) is one of the key technologies for such optical fiber communication networks. WDM employs multiple wavelengths in a single fiber to transmit in parallel different communication protocols and bit rates. Transmitting several channels in a single optical fiber at different wavelengths can multi-fold expand the transmission capacity of the existing optical transmission systems, and facilitate many functions in optical networking. An international standard wavelength grid has been suggested by ITU (International Telecommunication Union) for the center wavelengths of WDM systems. Different technologies have thus been developed to divide or combine channels or subgroups of channels in the ITU grid.
From a terminology's viewpoint, a device that multiplexes different wavelength channels or groups of channels into one fiber is a multiplexer, and a device that divides the multiplexed channels or groups of channels into individual or subgroups of channels is a demultiplexer. Specifically, a multiplexer combines several channels of optical signals into a single signal, or in reverse a demultiplexer separates a single multichannel signal into several individual channel signals, such multiplexer/demultiplexer is referred to a multiplexing/demultiplexing module, or simply multiplexer or demultiplexer.
Known devices for multiplexing/demultiplexing have employed, for example, diffraction gratings, arrayed waveguide gratings and various types of fixed or tunable filters. Gratings typically require complicated alignment systems and have been found to provide poor efficiency and poor stability under changing ambient conditions. Fixed wavelength filters, such as interference coatings, can be made substantially more stable, but transmit only a single wavelength or wavelength band.
U.S. Pat. No. 5,583,683 to Scobey discloses an optical multiplexing device that spatially disperses collimated light from a fiber optic waveguide into individual wavelength bands, or multiplexes such individual wavelength bands to a common fiber optic waveguide or other destination. An optical block has an optical port for passing multiple wavelength collimated light to be demultiplexed. Multiple ports are arrayed in spaced relation to each other along a multiport surface of the optical block to receive respective the individual wavelength bands. With respective collimators that must be precisely coupled to the multiple ports, the optical multiplexing device can be bulky, expensive and susceptible to varying ambient conditions (e.g. temperature and vibrations).
Another optical multiplexing device, called compact WDM device, is to mount all WDM filters and collimating means on a common substrate. As shown in FIG. 1, where each of the WDM filters 102–109 and collimators 0–9 can be tuned separately and fixed to the common substrate. Thus, when a multiplex optical signal including, for example, eight different channels or wavelengths (e.g. λ1 λ2 λ3 λ4 λ5 λ6 λ7 λ8), is coupled to a first collimator (i.e., Collimator 0), the optical signal is coupled to the filter 102 that transmits only λ1 and the rest is reflected to the filter 103 that transmits λ2 and reflects the rest. The rest of the signal continues to travel through the rest of the filters 104–109 and every time the signal hits a filter, a wavelength is filtered out and coupled to a collimator for output.
However, the compact WDM devices encounter serious problems with assembling tolerance. Analysis and experiment have proved that the assembly error that most sensitively degrades performance of the compact WDM devices is the amplified filter tilting error propagation. This situation can be understood from FIG. 2. If the first filter has a lateral tilt error of Δθ, the remaining signal hitting the next filter, or simply the next drop channel λ2 will experience two errors: the position has a lateral displacement ˜Δθ·l, and the incident angle has an error of Δθ. As a result, the drop channel λ2 suffers both in insertion loss and the central wavelength. For the next drop channel λ3, however, the performance degradation is amplified since the beam reflected from filter tilt is 2·Δθ and the position lateral shift is 2·Δθ·l. As the beam cascading further down, the subsequent drop channel will suffer even more degradation.
In summary, for the nth drop channel, the incident beam will result in an angular error of 2·Δθ angular and a lateral shift error of (n−1)·Δθ·l. This problem is significant in WDM modules configured for a large number of channels, where a minimal angular tilt of a filter, especially those used in the first few channels, will create amplified beam position deviation, both in lateral position and in angle, resulting in significant insertion loss.
There thus has been a need for compact WDM modules that minimize the problem of error amplification as discussed above and provide high tolerance for manufacturing with high yield.